1. Field of the Invention
The present invention relates to chargers for batteries and, more particularly, to extracting the maximum power available from a variable energy source to charge the batteries.
2. Description of Related Art
The present invention is directed to apparatus and a method for obtaining maximum power from an energy source, such as a photo voltaic panel, an array of photo voltaic panels, or a windmill. Although the invention is directed to only these two energy sources, the apparatus and methodology presented could be used in conjunction with any constant or variable output electrical energy source. To obtain the maximum power from an energy source requires two items. First, a methodology is required to determine the operating voltage and current that yields the maximum power that can be extracted from the energy source. Second, a circuit is required to extract the energy at the specified voltage and current. The circuit must be capable of accepting a wide range of voltage inputs and a wide range of current inputs. The circuit must also be able to maintain the input voltage and current at the values determined by the methodology to yield maximum energy extraction from the energy source.
Several methodologies and attendant circuitry have been used in the prior art for extracting maximum power from photo voltaic panels. The algorithms seek a voltage and current from the panels that result in maximum power being extracted from the panels These methodologies are summarized below with reference to illustrative U.S. patents:
a) Differentiation--U.S. Pat. No. 3,384,806 discloses use of the derivative of the output power versus time. A small sinusoidal signal is added to the control voltage of a pulse width modulation (PWM) controlled buck regulator and the time derivative of the output power is observed. Depending on the value of the derivative, a correction to the PWM control signal is obtained to yield maximum power extraction. A problem with this methodology is that it does not observe the entire current-voltage (I-V) characteristic of a photo voltaic panel and can lock on to a point that is a local maximum rather than an absolute maximum.
b) Open Circuit Voltage--In U.S. Pat. No. 4,873,480 and U.S. Pat. No. 4,604,567, the open circuit voltage of a photo voltaic panel or an array of panels is treated as directly being proportional to the voltage of the panels where maximum power can be extracted. The signal obtained from the open circuit voltage can then control a circuit that uses the signal to determine how much power to extract from the photo voltaic panels. The problem with this methodology is that the selection of the maximum power point does not consider the power extracted from the panels. It assumes that a given open circuit voltage accurately determines the operating point for maximum power extraction. If the operating point is incorrect or slightly inaccurate, operation at the maximum power point will not occur since the power extracted is not actually measured.
c) Tracking Cell--Many methodologies use a separate tracking cell to measure the amount of insolation incident on a photo voltaic array; note U.S. Pat. Nos. 4,873,480 and 3,696,286. The tracking cell yields information that could be used to select the maximum power point; that is, the open circuit voltage of a tracking cell is used. This methodology assumes that the cell has identical properties to each cell in a photo voltaic array; the open circuit voltage of the cell is treated as proportional to the open circuit voltage of the photo voltaic array. The open circuit voltage can then be used to select the maximum power point. If the assumption made is not correct, maximum power will not be drawn out.
d) Dithering--U.S. Pat. No. 5,327,071 discloses a switching circuit that controls the power extracted from a solar panel. Typically, the power is controlled by pulse width modulation (PWM) and a control input is available for controlling the power drawn from the energy source. The dithering methodology works as follows. The present operating power is recorded. The control signal is then increased or decreased (dithered) by a certain amount. The power at the new point is observed. By comparing the power at the original operating point to the power at the dithered points, a new maximum energy extraction operating point can be found, or the present operating point can be retained. A problem with this methodology is that it does not observe the entire I-V characteristic of a photo voltaic panel(s) and can lock on to a point that is a local maximum rather than an absolute maximum. U.S. Pat. No. 5,654,883 discloses use of a dithering method that examines m points above and below a present operating point. The power at each point is compared and a new maximum power operating point is selected. By examining m points, this method helps reduce the problem of finding a local maximum rather than an absolute maximum. However, it cannot eliminate the problem since it only examines m points rather than the entire I-V characteristic.